1. Field of the Invention
The invention relates generally to the field of electrical transformers of the type used in power networks for the generation, transmission and distribution of electrical energy and, more particularly, to a circuit arrangement for reducing a unidirectional flux component in the soft-magnetic core of a transformer.
2. Description of the Related Art
It is known undesirable injection of direct current may occur for different reasons in networks for generating, transmitting and distributing electrical power, e.g., caused by power electronic switching units in the network. Such direct current, hereinafter also termed a DC component, results in a unidirectional flux component causing asymmetrical saturation of the magnetic core material of a transformer. This saturation increases the losses and operating noise of the transformer. Another possible cause are “geomagnetically induced currents” (GIC) that are an issue particularly for high-power transformers, because the no-load losses and no-load noise are significantly increased. Depending on the design of the transformer, even a very small DC component of a few 100 mA can increase the emission of operating noise by 10 to 20 dB. In the case of GIC, DC components of up to 50 A can occur. A significant increase in losses of some 20-30% may arise. Local heating in the transformer can severely reduce the useful life of the winding insulation.
Various methods and devices for reducing a unidirectional flux component in the core of a transformer are known. For example, EP 2 622 614 B1 describes the use of a switching unit to inject a compensation current into a compensation winding magnetically coupled to the transformer core. The effect of the compensation current counteracts the unidirectional flux component. Controlled valves, such as thyristors, are proposed for implementing the switching unit. However, the use of controlled semiconductor switches is currently limited in practice to around 690 V because of their maximum permissible voltage or, rather, power dissipation. However, in the case of transformers used in the context of high-voltage DC transmission (HVDCT), the voltage induced in the compensation winding can be well above these limit values and indeed exceed 8 kV. The practical use of thyristors in this high-voltage range is not only limited, but also requires a considerable outlay for cooling equipment to dissipate the switching losses. Moreover, comparatively complex control circuits are required for controlling controlled valves, which adversely affects reliability. For practicable unidirectional flux compensation, similar requirements apply as for power transformers themselves, e.g., the compensation device shall have a simple design and must provide low-maintenance operation over several decades.